Use websites like gasbuddy.com to find the cheapest prices. Use a good GPS to keep from wasting gas getting from A to B.
Always stay below 65 to get better mileage. Check tire pressures often and keep your engine tuned up properly.
Stay back from other vehicles and look ahead for traffic slowdowns and red lights so you can keep from braking much.
Use AC only when necessary. Don't pack anything heavy that you don't need on the trip.